1. Field of the Invention
The invention relates in general to transport refrigeration systems, and more specifically to such systems having heating and cooling cycles which utilize hot compressor discharge gas.
2. Description of the Prior Art
Transport refrigeration systems for conditioning the loads of trucks and trailers have cooling, null and heating modes. The heating mode includes a heating cycle for controlling load temperature to a set point, as well as a heating cycle for defrosting the evaporator coil. When the system switches from a cooling or null mode into a heating cycle, hot compressor discharge gas is diverted by suitable valve means from the normal refrigerant circuit which includes a condenser, receiver, expansion valve, evaporator, and accumulator, to a circuit which includes the compressor, evaporator and accumulator.
To make more liquid refrigerant available during a heating cycle, a normal prior art procedure pressurizes the receiver with the hot compressor discharge gas to force liquid refrigerant out of the receiver and into the refrigerant cooling circuit. A bleed port in the expansion valve allows this liquid to flow into the evaporator during the heating cycle, to improve heating or defrosting capacity.
U.S. Pat. No. 4,748,818, which is assigned to the same assignee as the present application, improved upon the normal prior art procedure by eliminating the pressure line to the receiver, and by connecting the output of the receiver to the accumulator during a heating cycle. While this allowed some refrigerant to flow from the condenser to the receiver, I found that a substantial amount of refrigerant was still being trapped in the condenser, especially at low ambients, e.g., below about +15.degree. F. (-9.44.degree. C.).